Luminescence based measurements and devices are currently widely used methods in different fields such as biology, chemistry, materials science and medicine. Strong luminescence intensity is one of the most important desired properties of luminophores for these applications, especially in luminescence sensors. There is a continuing need for increasing luminescence sensitivity in biological research. However, detection and sensitivity in general is limited by the luminescence quantum yield and photostability of the probe. It is possible to design and synthesize luminophores with desired spectral properties. However, it is difficult to design luminophores with desired luminescence intensity.
Nearby conducting metallic particles, colloids, and surfaces are known to significantly influence the emission of vicinal luminophores.5-34 A luminophore near the metal surface is strongly quenched by the metallic surface but is enhanced when it is beyond the quenching region. This enhancement depends on the increase of the intrinsic decay rate of the luminophore, which can be described by lifetime. A reduction in lifetime occurs simultaneously with an increase in intensity. For example, shorter lifetimes for luminophores in proximity to silver nanoparticles coupled with enhanced emission intensities, has been reported in many publications.
Surface plasmon resonance (SPR) wavelength, one of the most important properties of nanostructures, dictates the choice of materials to be used for luminescence enhancement. SPR has been widely used as a quick and accurate detection of various physical, chemical, and biochemical parameters. In SPR, a metal dielectric interface supports an electromagnetic wave called a surface plasmon wave, which is a p=polarized wave that propagates along the interface. When the p-polarized light with a propagation constant equal to that of the surface plasmon wave is incident on such a metal dielectric interface, a strong absorption of light takes place. Surface roughness is known to provide a pathway for the coupling of incident light to surface plasmons and the creation of far-field radiation from the plasmons. Light intensity of nanoparticles at near field is strongly dependent on the SPR wavelength of the metal nanostructures. Spectral overlap between the absorption and emission spectra of luminophore and SPR spectra of metal nanoparticles is very important for optimum luminescence enhancement.
Luminophores in the excited state undergo near-field interactions with the metal nanoparticles to create plasmons. These plasmons radiate away from the nanoparticle to increase luminescence enhancement of luminophores. Excited luminophores that are in close proximity to metal nanoparticles can induce dipoles in the metal nanoparticles which under certain conditions radiate the photophysical properties of the luminophore. The efficiency of luminophore coupling to surface plasmons combined with their high efficiency to radiate produces luminophore-metal systems that display high luminescence quantum yields combined with reduced lifetimes.
Though the phenomena of metal enhanced luminescence (MEL) was originally presented in the 1980s, the demonstration and applications of MEL are largely a new field of investigation. MEL involves the interactions of luminophores with metallic nanoparticles which results in luminescence enhancement, increased photostability, decreased lifetimes due to increased rates of system radiative decay, reduced blinking in single molecule fluorescence spectroscopy, and increased transfer distances for fluorescence resonance energy transfer. The extent of MEL depends on the size, shape and dielectric constant of the nanoparticles which decide the surface plasmon resonance and scattering properties of nanoparticles and also on the separation distance between the metal and the fluorophore.
Luminescence enhancements ranging from tens- to hundreds-fold in signal intensity have been reported in the literature.4, 6, 8, 12, 14, 25, 35-37 Different applications of metal enhanced luminescence and from different metallic nanoparticles have been reported in recent literature.8-11, 14, 15, 20, 26, 27, 33, 34, 38 
At the vicinity of conducting metallic nanoparticles such as silver (Ag) and gold (Au), the emission intensity of luminophores is known to be significantly influenced.1-4, 39, 40 MEL has been studied mostly using silver nanoparticles5-9, 11-13, 15, 18, 33, 37 due to their intense and narrow SPR peaks. The Ag based sensor is known for its narrow spectral width but is chemically unstable and is highly vulnerable to oxidation when in liquid or gaseous environments. Gold nanoparticles are known to both quench and enhance luminescence depending on the fluorophore-particle separation distance, molecular dipole orientation with respect to particle surface, and size of the nanoparticles.24,25,41 Relatively smaller (typically less than 30 nm) gold nanoparticles quench fluorescence emission due to non-radiative transfer from the excited states of luminophore molecules to the gold nanoparticles.41 Larger gold nanoparticles can enhance luminescence due to the increased contribution of nanoparticle scattering.24,25 
MEL has primarily been studied in the visible —NIR wavelength region given most of the studies have been performed using silver or gold. The problem with this is that many widely used fluorophores absorb or emit at ultraviolet wavelengths. Recently, other metals, such as copper and aluminum, have been reported to enhance luminescence.28, 34, 39 But, due to the higher ohmic losses, the MEL effect is not as pronounced in Cu and Al as it is in Ag or Au. However, fluorescence does have possibilities to be enhanced in the ultraviolet-blue region of the spectrum using metals such as aluminum and copper. Recently zinc oxide (ZnO) nanorods platforms have been reported to enhance the luminescence intensity significantly, from commonly utilized fluorophores in immunoassays.36 
Sharma et. al. examined sensitivity, signal-to-noise ratio (SNR), and operating range for different bi-metallic nanoparticles for use in fiber optic SPR based sensors. Specifically they examined gold (Au), silver (Ag), copper (Cu), and aluminum (Al). They found that Cu has a slightly broader SPR curve than Al and its resonance wavelength is longer. Ag follows the same basic pattern and has a broader SPR curve and a higher resonance wavelength than either Cu or Al. Au has the broadest SPR curve and the highest resonance wavelength. They determined that no single metal nanoparticle is able to provide reasonable values for all three performance parameters including sensitivity, SNR and operating range simultaneously. However bi-metallic nanoparticles are able to show significantly high values for these three parameters.39, 40 
Using nanoparticle platforms, it is possible to increase the quantum yield of weakly luminescent probes by modifying their radiative decay rate to increase their emission efficiency, or by coupling the emission with far field scattering. The emission intensity of luminophores with nearly unit quantum yield can also be improved by enhancing their absorption through increasing the local electric field. The absorption and emission peaks of any luminophore can be predicted by analogy of known luminophores. Light intensity of nanoparticles at near field is strongly dependent on the surface plasmon resonance (SPR) wavelength of the metal nanostructures. SPR wavelength, one of the most important properties of nanostructures, dictates the choice of materials to be used for luminescence enhancement.
Tam et al.29 found that the enhancement is optimal when the plasmon resonance wavelength of the nanoparticles is tuned to the emission wavelength of the low quantum yield luminophores. The luminescence enhancement is largest when the emission wavelength is slightly red-shifted from that of the plasmon resonance12. By tuning the position of the SPR peak of the nanoparticles over a wide range of wavelengths, metal enhanced luminescence (MEL) can be extended to a wide range of luminophores. It appears that the optimal location of the SPR peak of nanoparticles is between the excitation and emission peaks of luminophores for maximum enhancement, as both excitation and emission rates can be enhanced in such a situation.12 
So far, MEL has been studied mostly on pure metal platforms. SPR wavelengths of pure metal nanoparticles can be tuned to different values by controlling several parameters, such as particle size, shape, particle-to-particle distance and surrounding dielectric medium12. However, it is easier to tune SPR spectra of alloy nanoparticles in a wide range of wavelengths as these offer additional degrees of freedom for tuning their optical properties by altering atomic composition and atomic arrangement. This enables development of specifically tailored nanoparticle platforms for MEL of a wide range of luminophores.
The SPR spectrum of Ag is more intense and narrower than that of Cu nanoparticles. The absorption peak attributed to SPR occurs at shorter wavelengths for Ag. Hence by modifying the composition and atomic arrangement we can tune both the breadth and the location of the peak of the SPR spectrum of Ag—Cu alloy nanoparticles. SPR peak wavelengths of Ag—Cu alloy nanoparticles can be tuned easily in the visible and near infrared region by changing only the annealing temperature. We have established simple and straightforward routes for the successful growth and fabrication of nanostructured platforms which can be effectively utilized to enhance the luminescence of any luminophore. We also provide insight into the effect of SPR on MEL. We use theoretical calculations based on models for calculating the excitation enhancement factor by local field effects and the emission enhancement factor due to radiative and nonradiative decay rate change.
The present invention demonstrates that SPR spectra of alloy nanoparticles can easily be tuned by manipulating only one experimental condition to result in maximum spectral overlap of the emission and absorption spectra of the luminophores with the SPR spectrum of the nanoparticles. Specifically we observed enhanced fluorescence emission from two thiol-reactive dyes, Alexa Fluor 594 and Alexa Fluor 488 at the proximity of Ag—Cu alloy nanoparticles.
The invention will improve luminescence sensor design and produce sensors having enhanced signal to noise ratio, resolution and detection sensitivity. An opportunity to enhance the luminescence of sensors will advance a wealth of biomedical and biochemical applications including single molecule detection, DNA sequencing, medical diagnostics, genomics. The improved luminescence will also facilitate fabrication of improved emissive devices, such as lasers or organic light-emitting diodes (OLEDs). Furthermore, this invention will extend the application of MEL to wide range of luminophores applicable for different field ranging from environmental analysis to biological research.